Three-Coordinate Nickel and Metal-Metal Interactions in a Heterometallic Iron-Sulfur Cluster

Enhanced Catalytic Activity via Rapid Two-Electron Transfer in Low-Spin Fe(II) Complex and Spin-State Dependent Proton Reduction Pathways

The growing interest in green hydrogen gas production has brought significant attention to the development of efficient proton reduction catalysts. A comprehensive understanding of proton and electron transfer processes within catalyst complexes is crucial for developing efficient catalysts. While the proton transfer process is influenced by the Brønsted acid used, electron transfer is an intrinsic property determined by the molecular orbitals and spin states of complexes. Complexes that rapidly transfer electrons are associated with high catalytic performance. In this study, we present a first example of low-spin FeII complex that utilizes the π* orbital of ligand for rapid two-electron transfer, resulting in exceptional catalytic performance for hydrogen gas evolution. The consecutive two-electron transfer rate was measured at 33.24 s-1, and in combination with proton transfer, the catalyst achieved an extraordinarily high turnover frequency (TOF) of 224,643 s-1 for hydrogen gas production. Conversely, a high-spin Fe(II) complex produced hydrogen gas at a relatively low TOF of 8848 s-1. These comparative experiments confirmed that the observed high catalytic efficiency is unique to the low-spin FeII complex, attributed to its distinct electron transfer mechanism.

Mimicking the CO2-Bound State of the [Ni,Fe]-CO Dehydrogenase

Complexes, where a doubly reduced CO2 2- (carbonite) ligand is spanned between a nickel(II) centre and a transition metal(II) ion (TM=Fe, Co, Zn) have been accessed. In non-coordinating solvents the carbonite ligand exhibits a flexible coordination behaviour as observed by NMR spectroscopy and supported by DFT calculations. In particular the [Ni-CO2-Fe] representative replicates the respective entity in an intermediate formed during CO2-conversion by the enzyme [Ni,Fe]-CODH in many ways (structure, spectroscopic properties, reactivity). Our investigations reveal that transition metal ions reduce the reduction potential of the carbonite unit but increase its tendency to undergo C-O bond cleavage. This may explain the choice of an iron(II) ion instead of a s- or p-block-based Lewis acid as part of the active site.

Nickel model complexes to mimic carbon monoxide dehydrogenase reactions

Biological CO2/CO interconversion catalyzed at the Ni/Fe heterobimetallic active site of anaerobic carbon monoxide dehydrogenases (CODHs) offers important insights for the design of efficient and selective synthetic catalysts for CO2 capture and utilization (CCU). Notably, this organometallic C1 interconversion process is mediated at a three-coordinate nickel site. Extensive research has been conducted to elucidate the redox and structural changes involved in substrate binding and conversion. The CO2-bound structure of CODH, in particular, has inspired many synthetic studies aimed at exploring key questions, concerning the choice of metal, the role of the unique iron (Feu), and the geometry and oxidation states of both Ni and Feu, as well as CO2/CO exchange mechanism. A better understanding of CODH chemistry promises to reveal and uncover fundamental principles for small molecule activation of first-row transition metal complexes. This mini-review focuses on three key aspects: (1) the coordination environment of the Ni centre in CODH, (2) bioinorganic Ni model systems that provide insight into the biological CO2/CO interconversion at the CODH active site, and (3) recent advances in CODH-inspired catalysis for selective CO2-to-CO conversion.

Diamidocarbene-derived palladium and nickel-sulfur clusters

Novel palladium and nickel-sulfur clusters were synthesized using a diamidocarbene-derived carbon disulfide ligand. Structural characterization revealed a tetranuclear metal-sulfur cluster geometry with each metal center exhibiting square-planar coordination. The ligand was redox-active, accommodating oxidation states ranging from 0 to -2.

Inserting Three-Coordinate Nickel into [4Fe-4S] Clusters

Metalloenzymes can efficiently achieve the multielectron interconversion of carbon dioxide and carbon monoxide under mild conditions. Anaerobic carbon monoxide dehydrogenase (CODH) performs these reactions at the C cluster, a unique nickel-iron-sulfide cluster that features an apparent three-coordinate nickel site. How nature assembles the [NiFe3S4]-Feu cluster is not well understood. We use synthetic clusters to demonstrate that electron transfer can drive insertion of a Ni0 precursor into an [Fe4S4]3+ cluster to assemble higher nuclearity nickel-iron-sulfide clusters with the same complement of metal ions as the C cluster. Initial electron transfer results in a [1Ni-4Fe-4S] cluster in which a Ni1+ ion sits outside of the cluster. Modifying the Ni0 precursor results in the insertion of two nickel atoms into the cluster, concomitant with ejection of an iron to yield an unprecedented [2Ni-3Fe-4S] cluster possessing four three-coordinate metal sites. Both clusters are characterized using magnetometry, electron paramagnetic resonance (EPR), Mössbauer, and X-ray absorption spectroscopy and supported by DFT computations that are consistent with both clusters having nickel in the +1 oxidation state. These results demonstrate that Ni1+ is a viable oxidation state within iron-sulfur clusters and that redox-driven transformations can give rise to higher nuclearity clusters of relevance to the CODH C cluster.

Mixed Valence {Ni2+Ni1+} Clusters as Models of Acetyl Coenzyme A Synthase Intermediates

Acetyl coenzyme A synthase (ACS) catalyzes the formation and deconstruction of the key biological metabolite, acetyl coenzyme A (acetyl-CoA). The active site of ACS features a {NiNi} cluster bridged to a [Fe4S4]n+ cubane known as the A-cluster. The mechanism by which the A-cluster functions is debated, with few model complexes able to replicate the oxidation states, coordination features, or reactivity proposed in the catalytic cycle. In this work, we isolate the first bimetallic models of two hypothesized intermediates on the paramagnetic pathway of the ACS function. The heteroligated {Ni2+Ni1+} cluster, [K(12-crown-4)2][1], effectively replicates the coordination number and oxidation state of the proposed "Ared" state of the A-cluster. Addition of carbon monoxide to [1]- allows for isolation of a dinuclear {Ni2+Ni1+(CO)} complex, [K(12-crown-2)n][2] (n = 1-2), which bears similarity to the "ANiFeC" enzyme intermediate. Structural and electronic properties of each cluster are elucidated by X-ray diffraction, nuclear magnetic resonance, cyclic voltammetry, and UV/vis and electron paramagnetic resonance spectroscopies, which are supplemented by density functional theory (DFT) calculations. Calculations indicate that the pseudo-T-shaped geometry of the three-coordinate nickel in [1]- is more stable than the Y-conformation by 22 kcal mol-1, and that binding of CO to Ni1+ is barrierless and exergonic by 6 kcal mol-1. UV/vis absorption spectroscopy on [2]- in conjunction with time-dependent DFT calculations indicates that the square-planar nickel site is involved in electron transfer to the CO π*-orbital. Further, we demonstrate that [2]- promotes thioester synthesis in a reaction analogous to the production of acetyl coenzyme A by ACS.

The interplay of covalency, cooperativity, and coupling strength in governing C-H bond activation in Ni2E2 (E = O, S, Se, Te) complexes

Dinickel dichalcogenide complexes hold vital multifaceted significance across catalysis, electron transfer, magnetism, materials science, and energy conversion. Understanding their structure, bonding, and reactivity is crucial for all aforementioned applications. These complexes are classified as dichalcogenide, subchalcogenide, or chalcogenide based on metal oxidation and coordinated chalcogen, and due to the associated complex electronic structure, ambiguity often lingers about their classification. In this work, using DFT, CASSCF/NEVPT2, and DLPNO-CCSD(T) methods, we have studied in detail [(NiL)2(E2)] (L = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane; E = O, S, Se and Te) complexes and explored their reactivity towards C-H bond activation for the first time. Through a comprehensive analysis of the structure, bonding, and reactivity of a series of [(NiL)2(E2)] complexes with E = O, S, Se, and Te, our computational findings suggest that {Ni2O2} and {Ni2S2} are best categorised as dichalcogenide-type complexes. In contrast, {Ni2Se2} and {Ni2Te2} display tendencies consistent with the subchalcogenide classification, and this aligns with the earlier structural correlation proposed (Berry and co-workers, J. Am. Chem. Soc. 2015, 137, 4993) reports on the importance of the E-E bond strength. Our study suggests the reactivity order of {Ni2O2} > {Ni2S2} > {Ni2Se2} > {Ni2Te2} for C-H bond activation, and the origin of the difference in reactivity was attributed to the difference in the Ni-E bond covalency, and electronic cooperativity between two Ni centres that switch among the classification during the reaction. Further non-adiabatic analysis at the C-H bond activation step demonstrates a decrease in coupling strength as we progress down the group, indicating a correlation with metal-ligand covalency. Notably, the reactivity trend is found to be correlated to the strength of the antiferromagnetic exchange coupling constant J via developing a magneto-structural-barrier map - offering a hitherto unknown route to fine-tune the reactivity of this important class of compound.

Opportunities for Insight into the Mechanism of Efficient CO2/CO Interconversion at a Nickel-Iron Cluster in CO Dehydrogenase

The reduction of CO2 with low overpotential and high selectivity is a crucial challenge in catalysis. Fortunately, natural systems have evolved enzymes that achieve this catalytic reaction very efficiently at a complex nickel-iron-sulfur cluster within carbon monoxide dehydrogenase (CODH). Extensive biochemical, crystallographic, and spectroscopic work has been done to understand the structures and mechanism involved in the catalytic cycle, which are summarized here from the perspective of mechanistic organometallic chemistry. We highlight the ambiguities in the data and suggest experiments that could lead to clearer understanding of the mechanism and structures of intermediates at the active-site cluster. These include parallel crystallography and spectroscopy, as well as the preparation of synthetic analogues that help to interpret structural and spectroscopic signatures.

Electronic isomerism in a heterometallic nickel-iron-sulfur cluster models substrate binding and cyanide inhibition of carbon monoxide dehydrogenase

The nickel-iron carbon monoxide dehydrogenase (CODH) enzyme uses a heterometallic nickel-iron-sulfur ([NiFe4S4]) cluster to catalyze the reversible interconversion of carbon dioxide (CO2) and carbon monoxide (CO). These reactions are essential for maintaining the global carbon cycle and offer a route towards sustainable greenhouse gas conversion but have not been successfully replicated in synthetic models, in part due to a poor understanding of the natural system. Though the general protein architecture of CODH is known, the electronic structure of the active site is not well-understood, and the mechanism of catalysis remains unresolved. To better understand the CODH enzyme, we have developed a protein-based model containing a heterometallic [NiFe3S4] cluster in the Pyrococcus furiosus (Pf) ferredoxin (Fd). This model binds small molecules such as carbon monoxide and cyanide, analogous to CODH. Multiple redox- and ligand-bound states of [NiFe3S4] Fd (NiFd) have been investigated using a suite of spectroscopic techniques, including resonance Raman, Ni and Fe K-edge X-ray absorption spectroscopy, and electron paramagnetic resonance, to resolve charge and spin delocalization across the cluster, site-specific electron density, and ligand activation. The facile movement of charge through the cluster highlights the fluidity of electron density within iron-sulfur clusters and suggests an electronic basis by which CN- inhibits the native system while the CO-bound state continues to elude isolation in CODH. The detailed characterization of isolable states that are accessible in our CODH model system provides valuable insight into unresolved enzymatic intermediates and offers design principles towards developing functional mimics of CODH.